Remove -b option from pbrun.

The -b option reads as follows:
` The target job is directed to ignore hangup signals. This is particularly
useful for running the target program in the background.`

If needed, '-b' can be added to become_flags

Squashed commit of the following:

commit f2c9f5c011ae8be610301d597a34bfba1a391e08
Author: Aaron Bieber <aaron@bolddaemon.com>
Date:   Mon Oct 17 10:58:14 2016 -0600

    remove pbrun flags

commit f402679ac177c931ad64bd13306f62512a14fcd6
Author: Aaron Bieber <aaron@bolddaemon.com>
Date:   Fri Oct 14 15:29:29 2016 -0600

    use Password: vs assword: for matching pbrun prompt

commit cd2e90cb65854c4cc5dd8773404e520d40f82765
Author: Aaron Bieber <aaron@bolddaemon.com>
Date:   Fri Oct 14 15:28:58 2016 -0600

    move -b to pbrun_flags

lib/ansible/playbook/play_context.py

@@ -518,8 +518,8 @@ class PlayContext(Base):
 
             elif self.become_method == 'pbrun':
 
-                prompt='assword:'
-                becomecmd = '%s -b %s -u %s %s' % (exe, flags, self.become_user, success_cmd)
+                prompt='Password:'
+                becomecmd = '%s %s -u %s %s' % (exe, flags, self.become_user, success_cmd)
 
             elif self.become_method == 'ksu':
                 def detect_ksu_prompt(b_data):